1. Field of the Invention
This invention relates to a breathing circuit. More particularly this invention relates to a coaxial breathing circuit in a respiratory therapy device.
2. Description of Prior Arts
Of the breathing circuits heretofore introduced for use in the administration of ventilation, there are a wide variety of the type to be used by connecting them to ventilators (otherwise called "respirators").
Conventional devices of the class mentioned above generally have been constructed so that an inhalation tube adapted to feed the patient with air or an oxygen-rich gas mixture in a compressed state via a ventilator through, or not through, a heated humidifier, and an exhalation tube adapted to release the gas exhaled by the patient into the ambient air either directly or indirectly via a ventilator or spirometer, are formed of two separate tubes except for a Y-piece serving as a mouthpiece.
The presence of an inhalation tube and an exhalation tube in a mutually separated form between the ventilator and the patient, however, causes an inconvenience in the handling of the device, adds to the complexity of medical equipment used on the patient undergoing the administration of ventilation and appended by a number of transfusion lines and monitor lines connected to the patient's body and, during a bronchial lavage, offers possible interference and even hindrance. To preclude the difficulties mentioned above, there has been suggested a breathing circuit wherein the inhalation and exhalation tubes are formed with a coaxial tube consisting of an external tube and an internal tube and, therefore, appear to be one tube (Japanese Published Unexamined Patent Application No. 150,893/1979). This circuit comprises a coaxial tube formed by coaxially arranging a straight inner tube and a usually corrugated outer tube, with the interior of the inner tube used for passage of the inhalation and the space between the outer tube and the inner tube used for passage of the exhalation. The patient side end of the outer tube is fastened to a connector provided with an opening for the passage of both incoming and outgoing air, and the inner tube is positioned near this air opening. The other ends of the outer tube and inner tube on an anaesthetic machine side are respectively fitted to a manifold, so that the anaesthetic machine will feed in and out the air via the orifices of this manifold which respectively communicate with the inner tube and the outer tube.
Such formation of the inhalation and exhalation tubes in a coaxial construction for use in the ventilator or anaesthetic machine offers the advantage that the handling of the device becomes easy. The use of the corrugated tube as an outer tube, however, is inevitably required in order for the passage to be prevented from an otherwise possible crush due to the so-called kinking. When the pressure is applied to the interior of the passage during the inhalation, the outer tube is consequently expanded mainly in its axial direction by this pressure. The end of the inner tube which falls on the patient side of the circuit is not fastened generally to the connector to which the outer tube is fixed. Consequently, the entire circuit is expanded by the rise of the inner pressure inevitably to increase the variation of the mechanical dead space.
When the circuit of this nature is used as a respiration circuit, there is a possibility that the increase and decrease of the mechanical dead space based on the overall elongation of the circuit will vary the amount of ventilation and keep it from reaching the expected level in accordance with increase and decrease of the inner pressure of the circuit, based on variation of air passage. There is entailed another disadvantage that throughout the course of the ventilation, the ventilator should be kept controlled for the adjustment of the amount of ventilation. When the aforementioned coaxial tube is put to use, it can be adapted for the ventilator if the ventilator itself is of the type incorporating therein necessary devices such as the exhalation valve. Otherwise, the adaptation is not obtained. This point forms a problem for the circuit under discussion.
Further, Japanese Published Unexamined Patent Application No. 150,893/1979 discloses an embodiment in which the inner tube is free in its axial direction at the connector on the patient side. In this construction, however, the inner tube is separated from the spacer and the whole circuit is expanded when the inner pressure is raised to a level over some tens of cm H.sub.2 O owing to the increase of the airway resistance and/or of the lung compliance. This embodiment has been demonstrated to be incapable of precluding the disadvantages mentioned above.
Generally in the administration of artificial respiration, it becomes necessary to dose aerosolized or a medication for the purpose of humidifying the inhalation, decreasing the viscosity of sputum in the patient's airway, or dilating the bronchus. Sometimes, therefore, a nebulizer (atomizer) is incorporated in the inhalation tube. In this case, to retain the efficiency of airsol inhalation intact and keep the patient from extra burden, the nebulizer is desired to be disposed at a position halfway in the length of the inhalation path and relatively close to the patient side. It is difficult, however, to insert the nebulizer only in the inhalation path of the coaxial circuit. No case has been known to the art in which the nebulizer is successfully inserted in the coaxial circuit of the construction described wherein above the product thus obtained is effectively used as a breathing circuit.
As a breathing circuit adaptable for a system which provides no exhalation valve for the ventilator as described above, there has been already proposed a manifold designed to provide the nebulizer and the exhalation valve in the exhalation tube (U.S. Pat. No. 3,826,255). Because of its particular construction, however, this manifold is intended for use with a breathing circuit containing two single-wall tubes instead of the coaxial tube. Thus, it cannot be adapted for the coaxial breathing circuit which enjoys the convenience of handling as described above. The applications found for the conventional coaxial tube breathing circuit have been limited.
British Patent Publication No. 2,029,703 discloses an anestherizing circuit which has both an inner and an outer tube fastened to the patient side connector. In this specification, the desirability of using corrugated tubes for both the inner and the outer tube is mentioned. It has been found that, when the circuit disclosed in this specification as exclusively designed for anesthetic is adopted as a breathing circuit, the use of corrugated tubes for the inner and the outer tube brings about rather undesirable results. The breathing circuit, by nature, is used for a much longer period than an anesthetizing circuit. Consequently, the inner pressure of the circuit is heightened as by the increase in the resistance of the airway and change of lung compliance, with the result that the whole circuit is expanded and the mechanical dead space is increased. Besides, the change in the resistance of the airway causes the volume of dead space to vary with elapse of time. In this case, since the inner tube restricts the expansion of the outer tube, the volume of the dead space is smaller than when the inner tube is not fixed to the patient side connector. It is nevertheless difficult to maintain the volume of ventilation constantly, because there is still the dead space change based on elongation of the corrugated tube itself. Since the circuit is primarily intended for the anesthetic circuit, the insertion of a nebulizer in the circuit is indicated or suggested nowhere in the specification of the British patent publication.
An object of the present invention, therefore, is to provide a novel breathing circuit.
Another object of the invention is to provide a coaxial breathing circuit which reduces the expansion and contraction of the entire circuit and the change of volume of the dead space to extremely small extents even when the inner pressure of the circuit extremely rises, and which limits to an extremely small extent the time-course variation of the volume of dead space due to the change in the resistance of the airway.
A further object of the invention is to provide a coaxial breathing circuit which is provided with a nebulizer and an exhalation valve.